Integrated circuit (IC) trimming involves the changing of a circuit component value to create a desired performance function. Usually the value of a resistance element is adjusted either step-wise or continuously. In the continuous adjustment a resistor is abraded, such as by means of an abrasive blast, or otherwise removed in part, such as by means of a vaporizing laser beam, to raise its resistance to a desired value. The parameter being adjusted is often monitored during the adjustment which can then be halted when the desired value is achieved. In the step-wise adjustment the step values are ordinarily previously known and often are fabricated to create a digitally related set of values. The parameter to be adjusted is first measured and the desired step determined. Then the desired step is achieved by whatever means is being employed. In some cases each resistor in a series string is shunted by a zener diode. When such a diode is subjected to a burnout pulse, in a process called zapping, it reverts to a short thereby shorting out the associated resistor.
U.S. Pat. No. 4,225,878, issued to Robert C. Dobkin on Sept. 30, 1980, and is assigned to the assignee of the present invention. This patent relates to zener diode zapping wherein the required number of IC bonding pads is reduced in number by employing a polarity-sensitive zapping procedure.
U.S. Pat. No. 4,412,241, issued to Carl T. Nelson on Oct. 25, 1983 and is also assigned to the assignee of the present invention. In this invention a combination of zener zapping and fuse blowing is employed to provide a plurality of resistance values. In a typical example, a pair of IC bonding pads is employed to either increase or decrease a nominal resistance to achieve any one of five discrete resistance values. If desired, the five discrete resistance values can be made to have substantially equal steps.
The teaching in the above two patents is incorporated herein by reference.
When trimming is to be applied to a voltage regulator or voltage reference supply, one trim is employed to achieve a particular voltage level. A second trim is then employed to adjust the temperature coefficient (tempco) of voltage. In typical prior art devices these two trims interact so that the adjustment includes a first adjustment of each parameter and then a second adjustment of at least one parameter to achieve the desired result.